Elastomeric materials have been long and extensively used in garments, both disposable and reusable products. These elastomeric materials may be attached to the disposable product by several methods. At one time, elastic was applied to the substrate by sewing. (See U.S. Pat. No. 3,616,770 to Blyther et al.; and U.S. Pat. Nos. 2,509,674 and RE 22,038 to Cohen). A newer method for attaching elastomeric material to a substrate is by use of an adhesive. (See U.S. Pat. No. 3,860,003 to Buell.) Welding, such as sonic welding, has also been used to attach elastomeric material to a disposable product. (U.S. Pat. No. 3,560,292 to Butter). Laminates having an elastomeric layer and a co-extensive skin layer have also been used. (U.S. Pat. No. 5,429,856 to Kruger et al.).
These methods of attachment present several problems. First is the problem of how to keep the elastic in a stretched condition while applying the elastic to the substrate. Another problem is that attachment of a ribbon of elastomeric material will concentrate the elastomeric force in a relatively narrow line. This may cause the elastic to pinch and irritate the wearer's skin. (See U.S. Pat. Nos. 3,860,003; 4,352,355; and 4,324,245 to Musek et al.; U.S. Pat. No. 4,239,578 to Gore; and U.S. Pat. Nos. 4,309,236 and 4,261,782 to Teed.) Other disadvantages of conventional attachment methods include speed, ease of manufacture, and cost. More importantly, difficulties may be encountered in maintaining a uniform tension on the elastic layer during its attachment to the substrate and also in handling the shirred article once the elastic layer is relaxed.
Heat-responsive elastomeric films overcome some of these detriments. Heat-responsive elastomers exist in two forms: a thermally-stable and a thermally-unstable form. The thermally-unstable form is created by stretching the material while heating near its crystalline or second phase transition temperature, followed by a rapid quenching to freeze in the thermally-unstable, extended form. The elastomeric film can then be applied to a disposable product, for example a diaper, and heated to shirr or gather the elastomeric material, thereby producing a thermally-stable form of the elastomeric material. Examples of heat-responsive elastomeric films are disclosed in U.S. Pat. No. 4,681,580 to Reising et al., U.S. Pat. No. 4,710,189 to Lash, U.S. Pat. No. 3,819,401 to Massengale et al., U.S. Pat. No. 3,912,565 to Koch et al., and U.S. Pat. No. RE 28,688 to Cook.
These polymers have several disadvantages. The first of these disadvantages involves the temperature to which the elastomeric material must be heated to stretch the material to its thermally-unstable form. This temperature is an inherent property of the elastomeric material. Therefore, the disposable product is often difficult to engineer because temperatures useful for the production of the overall product may not be compatible with the temperature necessary to release the thermally-unstable form of the elastomer. Frequently, this temperature is rather high and can be detrimental to the adhesive material used to attach the various product layers. Another drawback to the use of heat-responsive elastomers is that they can constrain the manufacturing process, rendering it inflexible to lot variations, market availability, cost of raw materials, and customer demands.
U.S. Pat. No. 4,820,590 to Hodgkin et al. describes an elastomeric blend of three components to reduce the temperature required for the material to resume its heat stable form. Additionally, GB Patent 2,160,473 to Matray et al. proposes an elastomer which will shrink at an elevated temperature, for example at or above 170° F. The advantageous features of these materials, compared to the heat-shrinkable materials discussed above, is that it does not require preheating during the stretching operation, but rather can be stretched at ambient temperatures by a differential speed roll process or by “cold rolling.”
Problems with use of these elastomers include difficulties inherent in applying a stretched elastic member to a flexible substrate such as a disposable diaper. Although some of the elastomers proposed have the advantage that they can be applied at ambient conditions in a highly stretched, unstable form, subsequent, often extreme, heating is required to release the thermally-unstable form to a contracted thermally-stable form. The temperature of this heat release is generally inflexible since it is determined at the molecular level of the elastomer. Thus, selection of materials for the disposable product which are compatible with this heating step is required.
Further, when individual heat activated elastic materials are used, the heat activation is generally accomplished by passing the garments through a heated air duct for a period of time. Since thermal heating must be transferred from an outer surface of the garment to inner portions of the garment, distribution of the activation means (i.e., thermal heat) throughout the garment takes considerable amounts of time and energy, resulting in an inefficient activation process. In such a configuration, the activation process typically takes several seconds, or even minutes, to elevate the temperature of the elastic material to a level at which activation takes place, causing the elastic material to retract and gather the garment. As a result, such heating processes can consume vast amounts of energy and undesirably result in slower manufacturing speeds.
What is needed in the art is a method of activating a shape deformation of a material within 1 second and without using an inefficient thermal heating activation process. What is also needed in the art is a method of activating a shape deformation of a material without substantially increasing the temperature of the material.